Fuel cells provide an environmentally friendly source of electrical current. One type of high temperature fuel cell (HTFC) used for generating electrical power is the solid oxide fuel cell (SOFC). The SOFC includes an anode channel for receiving a flow of fuel gas, a cathode channel for receiving a flow of oxygen gas, and a solid electrolyte which is a ceramic membrane conductive to oxygen ions and separates the anode channel from the cathode channel. Oxygen in the cathode channel dissociates to oxygen ions, which cross the electrolyte to react with hydrogen in the anode channel to generate a flow of electrons. While hydrogen is a preferred fuel gas for efficient SOFC operation, the fuel gas may be a hydrocarbon which reacts in the anode channel either by direct oxidation or to generate hydrogen by steam reforming and water gas shift reactions. As hydrogen is consumed, carbon monoxide may be oxidized directly or may be shifted by steam to generate additional hydrogen. Carbon dioxide and water vapor are produced in the anode channel by oxidation of fuel components. Typical operating temperature of solid oxide fuel cells is about 500° to about 1000° C.
Another type of high temperature fuel cell used for generating electrical power, particularly envisaged for larger scale stationary power generation, is the molten carbonate fuel cell (MCFC). The MCFC includes an anode channel for receiving a flow of hydrogen gas (or a fuel gas which reacts in the anode channel to generate hydrogen by steam reforming and water gas shift reactions), a cathode channel for receiving a flow of oxygen gas, and a porous matrix containing a molten carbonate electrolyte which separates the anode channel from the cathode channel. Oxygen and carbon dioxide in the cathode channel react to form carbonate ions, which cross the electrolyte to react with hydrogen in the anode channel to generate a flow of electrons. As the hydrogen is consumed, carbon monoxide is shifted by steam to generate additional hydrogen. Carbon dioxide and water vapor are produced in the anode channel by oxidation of fuel components, and by reduction of carbonate ions from the electrolyte. Typical operating temperature of molten carbonate fuel cells is about 650° C.
For stationary power generation, hydrogen may be generated from natural gas by steam reforming or partial oxidation, and particularly by direct internal reforming within the anode channel, to produce “syngas” comprising a mixture of hydrogen, carbon monoxide, carbon dioxide, steam and some unreacted methane. As hydrogen is consumed in the fuel cell anode channel, much of the carbon monoxide reacts with steam by water gas shift to generate more hydrogen and more carbon dioxide. Other carbonaceous feedstocks (e.g. heavier hydrocarbons, coal, or biomass) may also be reacted with oxygen and steam to generate syngas by partial oxidation, gasification or autothermal reforming.
While the fuel cell may be operated on hydrogen or syngas that has been generated externally from a fossil fuel, efficient thermal integration between a high temperature fuel cell and an external fuel processing system may be difficult to achieve, since the fuel cell stack generates excess heat remote from the endothermic heat demand of fuel processing.
In order to achieve benefits of simplicity and better thermal integration, most SOFC developments for natural gas as the hydrocarbon fuel have contemplated internal reforming, in which the steam methane reforming reaction is conducted within the anode channel. A conventional SOFC anode material is nickel cermet with yttria stabilized zirconia (Ni-YSZ), which is an active catalyst for steam methane reforming. However, the nickel cermet is also catalytic for carbon deposition which must be avoided, typically by operating with a sufficiently high steam/carbon ratio with the adverse consequence that the excess steam degrades the SOFC voltage output. Under typical SOFC operating conditions, the steam reforming reaction will be substantially complete within about the first 20% of the anode channel, resulting in excessive cooling of that zone, which degrades performance and causes thermal stresses that may damage the SOFC stack. To ameliorate these problems, it is standard practice to include a pre-reformer which may achieve about 30% conversion of the steam reforming reaction upstream of the anode channel entrance. The pre-reformer also reduces the risk of carbon deposition within the anode, by accelerated reforming or methanation of the more reactive higher hydrocarbon components. The pre-reformer may be an externally heated steam reformer or an autothermal reformer based on partial oxidation.
Further simplification could be achieved if the hydrocarbon fuel could be oxidized directly within the SOFC anode channel, without addition of steam. Thus, Barnett et al (U.S. Pat. No. 6,214,485 B1) have used a nickel yttria doped ceria (Ni/YDC) cermet anode without carbon deposition on methane at temperatures below 800° C. Gorte et al (U.S. Patent Application Publication US 2001/0053471 A1) have used copper ceria over porous yttria stabilized zirconia cermets (Cu/CeO2/YSZ) to demonstrate operation on a range of hydrocarbon fuels without carbon deposition. H. Kim et al (J. Electrochem. Soc. 149, p. A247, 2002) used copper-nickel alloy ceria cermets for direct SOFC oxidation of methane.
While the above developments of advanced SOFC anode materials can enable direct oxidation of dry methane and some other hydrocarbons, the anode catalytic activity for hydrocarbons is much inferior to that for hydrogen. Hence, severe anode over-voltages for direct oxidation of hydrocarbons are typical, and higher performance (least activation polarization) would always be expected with hydrogen. It is noteworthy that activity of a Cu-ceria-YSZ cermet (R. Gorte et al, Adv. Mater. 2000, 12, p. 1465, 2000) increases in the order of methane<butane<hydrogen, indicating that the LPG components of natural gas should be oxidized more readily than methane, as expected owing to their greater reactivity than the more stable methane molecule.
The lower heat of combustion of a fuel usefully defines the energy (enthalpy change of the reaction) that may be generated by oxidizing that fuel. The electrochemical energy that can be generated by an ideal fuel cell is however the free energy change of the reaction, which is smaller than the enthalpy change. The difference between the enthalpy change and the free energy change is the product of the entropy change of the reaction multiplied by the absolute temperature. This difference widens at higher temperatures, so higher temperature fuel cells inherently convert a lower fraction of the fuel energy to electrical power at high efficiency, while a larger fraction of the fuel energy is available only as heat which must be converted to electrical power by a thermodynamic bottoming cycle (e.g. steam or gas turbine plant) at lower efficiency.
For both SOFCs and MCFCs, accumulation of reaction products (carbon dioxide and steam) on the fuel cell anode opposes the electrochemical reaction, so that the free energy is reduced. Higher partial pressure of oxygen over the cathode, and higher partial pressure of hydrogen over the anode, drive the reaction forward so that the free energy is increased. Unfortunately, the reaction depletes the oxygen in the cathode channel and depletes hydrogen in the anode channel while rapidly increasing the backpressure of carbon dioxide as a diluent in the anode channel. Hence the free energy change is reduced, directly reducing the cell voltage of the fuel stack. This degrades the electrical efficiency of the system, while increasing the heat that must be converted at already lower efficiency by the thermal bottoming cycle.
The free energy change is simply the product of the electromotive force (“E”) of the cell and the charge transferred per mole by the reaction (“2F”), where the factor of two reflects the valency of the oxygen ion. The following Nernst relation for a SOFC expresses the above described sensitivity of the electromotive force (open circuit voltage, or Gibbs free energy expressed as electron-volts per electron) to the partial pressures of the electrochemical reactants in the anode and cathode channels, where the standard electromotive force (“Eo”) is referred to all components at standard conditions and with water as vapor.
  E  =            E      o        -                  RT                  2          ⁢          F                    ⁢              ln        ⁡                  [                                    P                              H2O                ⁡                                  (                  anode                  )                                                                                    P                                  H2                  ⁡                                      (                    anode                    )                                                              ·                              P                                  O2                  ⁡                                      (                    cathode                    )                                                  0.5                                              ]                    
The open circuit voltage is elevated by a high ratio of hydrogen to steam over the anode and by increased partial pressure of oxygen over the cathode. At finite working current density, the operating voltage is determined by subtracting ohmic resistance losses, activation polarization and concentration polarization from the open circuit voltage.